This invention relates to the pumping of volatile liquids and more particularly to pumps operating at low net positive suction head (NPSH) while delivering at its output a relatively large quantity of liquid at a relatively high pressure.
The prior art has recognized significant difficulties in pumping liquids where the liquid pressure (suction pressure) at the pump inlet is very low. The difficulties that arise with low inlet pressure pumps are associated with cavitation or boiling of the liquid being pumped. If the ambient pressure applied to the liquid is below its vapor pressure, the liquid being pumped will boil or cavitate. Cavitation is particularly troublesome at the inlet to the vanes of a centrifugal impeller. At this point in a centrifugal pump, there is an initial depressurization zone through which the fluid must traverse before the impeller vanes become effective in the process of raising the static pressure of the liquid being pumped. Boiling or cavitation will occur in this depressurization zone of the pump if the liquid pressure drops below the vapor pressure of the liquid being pumped. Such cavitation has two undesirable effects. First, a cavitating liquid has an increased chemical activity with the materials of which the pump is made. Rapid oxidation with the pump materials may occur. Secondly, cavitation may create relatively high pressures on various portions of the pump structure with the possible damage to the mechanical structure thereof.
The pressure reduction at the impeller vane inlet becomes acutely greater with increased flow rate and rotational speed of the pump, leading to critical cavitation problems for a number of pumping applications. Liquid sodium pumps for nuclear reactor coolant service is just one example of this. Because of the explosive nature of sodium when exposed to the atmosphere, safety precautions require that penetrations into the liquid metal system (i.e., where a pump shaft penetrates the liquid metal system) occur at a very low pressure point in the system. This is because a mechanical shaft seal, to prevent liquid sodium leakage into the atmosphere, is feasible for a maximum sodium pressure that is only slightly greater than atmospheric pressure. Therefore, the liquid sodium pump must operate at a very low pressure. Also, these pumps are required to handle large volume rates of sodium. These two system design conditions force the liquid sodium pump to operate at or close to a critical cavitating mode.
Thus, it is desired to provide a pump capable of operating at very low NPSH conditions. A reduction in NPSH can be achieved by lowering the rotational speed of the pump's impeller. The following equation relates NPSH to the flow rate of the pump and the rotational velocity of the pump's impeller: ##EQU1## where, NPSH=required net positive suct. head in feet
N=pump speed in rpm PA1 Q=pump flow in gpm ##EQU2## Reducing the impeller speed to satisfy a low NPSH requirement requires further modification of the pump design to meet typical pump head and flow design criteria. For example, to maintain an output pressure at a desired high level, it may be necessary to increase the dimensions of the pump's impeller and housing.
Alternatively, the pump inlet pressure and NPSH can be raised by returning a portion of the pump discharge to the pump inlet. However, such a pump modification reduces the overall efficiency of the pump. As a result, such a modification is not a practical solution to obtain low NPSH while providing a high pressure output.
The prior art has further suggested that two pumping units be coupled in series. The first, or booster pump, operates at a relatively low impeller speed to raise the fluid pressure head to a sufficient level that the second, or main pump, can operate at a relatively high impeller speed considering impeller size, overall pump efficiency, and the desired relatively high pressure output. However, the use of a separate booster pump and a main pump increases the space required and the resultant cost of such a configuration.
In a FINAL REPORT entitled "INDUCER DYNAMICS FULL-FLOW, FULL-ADMISSION HYDRAULIC TURBINE DRIVE, dated Aug. 24, 1969 by Farquhar et al. (NASA CR-72566; AGC-9400-18), there is disclosed a pump or inducer designed to pump a highly volatile liquid such as liquid rocket fuel. The described pump includes an inlet for receiving the volatile liquid. A first stage comprises a low speed inducer that is driven by a hydraulic turbine, as will be explained. A high speed rotor is disposed next within the flow path and is driven by a suitable motor at a relatively high speed. The high speed rotor increases the pressure of the volatile liquid and directs the liquid to drive a plurality of fins of the hydraulic turbine placed directly in the flow path. The fins are in turn connected by a coupling disposed outside of the flow path directly to the low speed inducer, whereby upon rotation of the turbine fins, the low speed inducer will rotate therewith. The speed of the turbine fins and, thus, the low speed inducer is dependent upon the angle with which the fins are mounted with respect to the liquid flow path. The liquid leaving the hydraulic turbine is directed to a main impeller, likewise coupled to the drive motor, before being discharged through an outlet. The problems with such a two-stage pump reside primarily with the use of turbine fins disposed within the primary flow path. First, because of the extra hydraulic losses associated with the inducer, this device should be designed for the minimum power consistent with producing no more than the minimum head required to prevent cavitation at the inlet to the high speed rotor. However, the turbine fins, disposed within the fluid path, significantly reduce the fluid pressure at the outlet of this pump. To compensate for this pressure drop, the high speed rotor must be driven at a relatively higher speed, thus, requiring a higher head inducer stage, with increased pumping losses, to prevent cavitation with the higher speed rotor. Secondly, the turbine fins, disposed in the primary fluid path, are effected by the vagaries of the primary fluid system. For instance, fluid power to the turbine fins can be effected by a change in the pump flow as a result of a change-over in system operating mode. This could cause a significant change in turbine speed with a resulting reduced inducer head and cavitation at the inlet to the high speed rotor. Also, the turbine fins would be subjected to unequal circumferential forces that can occur in the primary fluid flow path within the pump. This could result in severe loads being transmitted to the turbine bearing support system. Finally, the set of turbine fins disposed in the primary fluid path is an additional rotating component that must be considered in the design of the primary fluid path within the pump. As a likely result, the design of the primary flow system within the pump may be detrimentally compromised to accommodate the turbine. Conversely, the design of the turbine is dependent upon the fluid flow conditions that are imposed by the primary system. A likely result of this is a turbine with performance that has been detrimentally compromised because it is required to make use of non-optimum fluid flow conditions that are imposed on it by the primary fluid system in the pump.
Therefore, what is needed is a pump employing a booster or low speed stage and a high speed stage that does not employ driving means disposed within the flow path for driving the low speed or booster stage and additionally ensures that the suction pressure at the input for the second or high speed stage is sufficiently high to avoid cavitation.